some questions on active transport & osmosis

[lboogy]

Hi everyone,

 

I hope this is the correct section of the forum - if it isn't please could someone advise which section is most appropriate?

 

I have a few questions regarding active transport & osmosis. With potato pieces in sucrose, the water moves from the potato to the sucrose as there are fewer water molecules in the sucrose. Can we assume from this that the potato has no need for the water that it is releasing via osmosis? If it needed it, it could use active transport techniques to reatain it's own water plus acquire more from the sucrose?

 

when we use a weaker sucrose we find that the potato does acquire extra water via osmosis. So, this it seems is determined by the concentration gradient - irrespective of what the potato actually needs itself? Thus if there is more water outside it will take it in, but if there is less water outside it will release some, and wont use active transport to get more?

 

Please help!

thanks

[ecoli]

Hi everyone,

 

I hope this is the correct section of the forum - if it isn't please could someone advise which section is most appropriate?

 

I have a few questions regarding active transport & osmosis. With potato pieces in sucrose, the water moves from the potato to the sucrose as there are fewer water molecules in the sucrose. Can we assume from this that the potato has no need for the water that it is releasing via osmosis? If it needed it, it could use active transport techniques to reatain it's own water plus acquire more from the sucrose?

 

when we use a weaker sucrose we find that the potato does acquire extra water via osmosis. So, this it seems is determined by the concentration gradient - irrespective of what the potato actually needs itself? Thus if there is more water outside it will take it in, but if there is less water outside it will release some, and wont use active transport to get more?

 

Please help!

thanks

 

 

 

Keep in mind that the potato has evolved to be able to thrive in a specific environment. Just because it's exposed to different environment, doesn't mean that it will be able to survive there. It doesn't really have the mechanisms to pump water because it doesn't really plan on being exposed to sucrose solution, this wouldn't really happen in a natural habitat.

[lboogy]

so, even in a lab environment, when the potato will release water if the sucrose is strong, even to the extent that potato will be left weaker and flexible etc...is this an indication that it isn't capable of active transport?

[Bluenoise]

No.

Activity transport requires machienery that transports specific things. There is no real molecular machinery to transport water. The movement of water across a membrane is usually accomplished by the movement of ions (Cl-, Na+, K+, etc) that results in a chance of osmotic potential across the membrane. Water is able to just kinda sneak across membranes as it pleases.

So a potatoe in a saturated solution of only sucrose and water would have to be able to import a vast amount of sucrose into itself to create an isotonic environment, as sucrose is the only solute available.

And this is beyond the scope of the potatoe.

Like E. coli said. Potatoes have evolved for a specific environment, a hole in the ground. This environment isn't saturated in sucrose, thus is has no need for the capacity to evolve such a mechanism.

[lboogy]

thanks Bluenoise & ecoli. I have just 1 further question if I may!?

 

We all know osmosis is dependant on the semi permeable membrane - but can the concentration gradient be said to actually cause the osmosis?

[fredrik]

Yes, given that you have a semipermeable membrane, the concentration gradient can be said to cause the osmotic flow, but i's the solvent concentration or "activity". Equilibrium is when the solvent is equally likely to diffuse in either direction and there is thus no net flow. Chemists would say the chemical potential of the solvent is equal on both sides of the membrane. Or it's rather the "solvent activity" that matters, because there can be multiple solutes, and they could be different at the different sides of the membrane. So the solvent diffusion is determined not by the solute gradient, buy the "solvent gradient".

 

Passive transport, diffusion and osmosis across membranes are controlled by what's thermodynamically and kinetically favourable. Compare with "communicating buckets", there is flow until the levels are equal.

 

There is also a kind of fascilitated transport that may change the kinetics but not the thermodynamics.

 

Active transport is the transport of molecules against an otherwise thermodynamic gradient, or transport of molecules that simply are impermeable to the membrane itself. Living organisms can power such active membrane transport channels in various ways. Regardless of mechanics, these active transports often make up a significant part of the cells "energy bill". This means you can pump uphills, by using a powered pump. Or just as complement the passive transport in cases when time is expensive.

 

Yeasts for example, can utilize various sugars. But each sugar har their own transport cost. This often means that sugars that require expensive transport mechanism giver lower biomass yields and their uptake is often repressed in the presence of "cheaper" food.

 

/Fredrik

[lboogy]

thanks Fredrik

[fredrik]

A simple specific living example of osmosis in relation to cells. Is that draining the cell on water is obviously a physiological stress conditions, and for example brewers yeast are regularly stressed in high gravity fermentations. Because the extracellulary sugar concentration is so high. Part of the physiological stress response to "fight" the osmosis, is that the yeast starts to synthesise glycerol in the cytosol, and this counteracts the osmosis. This part of why high gravity wines and beers, tend to have higher residual levels of glycerol giving an additional sweetness. But as with most stresses in cells, there are several responses, both long term and short term responses. One long term response to osmotic stress is also a change in the fatty acid composition in the membrane lipids as it changes the physical properties of the membrane.

 

The chemistry and biology of various membranes is one of many keys topics to living organisms. Since my other hobby is beer fermentations and brewers yeast I've spend some time to analyze the logic of these things a bit.

 

/Fredrik